1. Field of the Invention
The present invention relates to a liquid crystal display performing display drive by using a TFT (thin film transistor) element.
2. Description of the Related Art
In recent years, as a display monitor in a liquid crystal television, a notebook personal computer, a car navigation system, or the like, there has been proposed a liquid crystal display which employs VA (vertical alignment) mode using vertically-aligned liquid crystal. In VA mode, liquid crystal molecules have a negative dielectric constant anisotropy, that is, the property in which dielectric constant in the long axis direction of the molecules is smaller than that in the short axis direction of the molecules. Thus, it is possible to realize the wide view angle in VA mode, in comparison with TN (twisted nematic) mode.
However, in the liquid crystal display using liquid crystal of VA mode, there is an issue that luminance in the case of viewing a display screen from the front direction differs from a luminance in the case of viewing the display screen from the oblique direction. FIG. 10 indicates the relationship between the gray scale (0 to 255 gray scale levels) and the luminance ratio (luminance ratio to the luminance in the 255 gray scale levels) in an image signal, in the liquid crystal display using liquid crystal of VA mode. As indicated with arrow P101 in the figure, luminance characteristics in the case of viewing the display screen from the front direction (Ys (0°)) are highly differs from luminance characteristics in the case of viewing the display screen from the oblique direction (Ys (45°)) (luminance is shifted high in the case of viewing the display screen from the oblique direction, in comparison with the case of viewing the display screen from the front direction). Such a phenomenon is called “higher luminance shift”, “wash out”, “color shift” and the like, and this is regarded as a major disadvantage of the liquid crystal display in the case where liquid crystal of VA mode is used.
To improve such a “higher luminance shift” phenomenon, a liquid crystal display is proposed in which a unit pixel is divided to a plurality of sub-pixels, and a threshold in each of the sub-pixels is varied (multi-pixel structure) (for example, U.S. Pat. Nos. 4,840,460 and 5,126,865).
FIG. 11 illustrates an example of the relationship between the gray scale of an image signal and a display state of each sub-pixel, in the multi-pixel structure. In the process when the gray scale is up (luminance increases) from 0 gray scale level (black display state) to 255 gray scale level (white display state), it is understood that the luminance of a part of the pixel (one of the sub-pixels) increases, and then the luminance of the other part of the pixel (other of the sub-pixels) increases. That is, since two thresholds are set in the multi-pixel structure, the variation in gamma characteristics is dispersed. Therefore, as indicated with arrow P102 in FIG. 10, for example, the “higher luminance shift” phenomenon is improved in the luminance characteristics of the 45-degree direction (Ym (45°)) in the multi-pixel structure, in comparison with the luminance characteristics of the 45-degree direction (Ys (45°)) in the typical pixel structure.
Here, the multi-pixel structure indicated in U.S. Pat. Nos. 4,840,460 and 5,126,865 is called HT (halftone gray scale) method by capacitive coupling, and difference in the electric potential between two sub-pixels is determined with capacity ratio.
On the other hand, there is also proposed the multi-pixel structure using two transistors, which is unlike the multi-pixel structure indicated in U.S. Pat. Nos. 4,840,460 and 5,126,865. In this multi-pixel structure, two sub-pixels are driven through different TFTs, connected to gate lines different from each other or data lines different from each other, respectively. That is, two sub-pixels are driven while being electrically perfectly independent from each other. In this multi-pixel structure, the gray scale data of each pixel is substituted according to a value in a predetermined look up table (LUT), or a plurality of types of reference voltages are used in a data driver. Thereby, the drive for each sub-pixel is performed.
In the typical pixel structure, it is proposed that a unit frame in the display drive is temporally divided to a plurality (for example, two) of sub-frames, and the desired luminance is divisionally expressed based on time by using the sub-frame having high luminance and the sub-frame having low luminance. With such a method, it is also possible to obtain the halftone effect similar to that of the multi-pixel structure, and it is known that the “higher luminance shift” phenomenon is improved.
In such a liquid crystal display of VA mode, when a voltage is applied, the liquid crystal molecules aligned vertical to a substrate responds to the voltage by tilting in the direction parallel to the substrate, with the negative dielectric constant anisotropy, and thereby light is transmitted. However, the direction where the liquid crystal molecules aligned vertical to the substrate tilt (director at the time of applying voltage) is arbitrary. Thus, with the voltage application, the alignment of the liquid molecules is disordered, and this is a major factor of deteriorating the response characteristics to the voltage.
As a control means (alignment controller) of the tilting direction in response to the voltage, the technique is disclosed such that insulating projections having inclined surfaces are provided at regular intervals on a substrate, and thereby the liquid molecules are aligned while tilting in a specific direction from the direction vertical to the substrate (MVA-LCD; for example, SID' 98, p. 1077, 1998). Moreover, as another control means (alignment controller), the method is proposed such that a slit (portion with no electrode) is provided in a part of a pixel electrode and a facing electrode. Thus, the voltage is applied to the liquid crystal molecules in the oblique direction (from the oblique electric field), and the alignment direction of the liquid crystal is controlled (PVA-LCD; for example, Asia Display, p. 383, 1998).
Moreover, as another control means (alignment controller), it is proposed that a floating electrode having a slip is provided on a control electrode (for example, U.S. Pat. Nos. 6,650,390 and 6,407,791). Even in the case where this method is used, like the projection in MVA-LCD and the slit in PVA-LCD, it is possible to apply the alignment control capability to the liquid crystal molecules, and the desired alignment control is possible.